I have a client, he receives a lot of spam from his ISP Mailserver.
Is there a way to limit spam using an OpenBSD Gateway with PF and Spamd at his place ? (His mailserver is ISP Mailserver, so he hasn't mailserver)

spamd(8) pretends to be a mail server, and temporarily blocks incoming mail from other mail servers it has not seen mail from before. Robot platforms that spew spam do not usually bother to retry. Spamd only blocks these robots, it does not stop spam sent from a real mail server.

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Once the mail has been retried a second time, spamd will tell PF to redirect the incoming mail to the real mail server.

Legitimate mail incoming from a previously unknown server will have to try three times to reach the real server.